Structural organization of the white pulp
The white pulp acts as a secondary lymphoid organ and appears as grain-like corpuscles—distinct inclusions within the red pulp. Its structure closely resembles lymph node architecture, except that the lymphoid tissue here forms exclusively around arterioles.
Several distinct zones can be identified within the structure of the white pulp, extending from the center of the blood vessel to the periphery:
- Periarterial lymphatic sheath (PALS / PALS(T)). A zone directly adjacent to the central artery. This is a thymus-dependent zone that serves as a niche for T cell localization. Functionally, the PALS is the analog of the paracortical zone of lymph nodes.
- Intermediate space. Surrounds the sheath and is occupied by a mixed population of lymphocytes of both classes (T and B cells).
- Lymphoid follicles. Located closer to the peripheral parts of the white pulp. They act as niches for T and B lymphocytes. Based on the presence of a germinal center and overall functions, they are completely identical to lymph node follicles.
- Marginal zone. Located at the very border between the white and red pulp.
- Marginal sinus. Finally separates the white pulp structures from the red pulp.
Features of the marginal zone
The marginal zone deserves special attention as a crucial peripheral compartment of the white pulp. Its cellular composition is dominated by B lymphocytes, primarily MZB cells.
The blood supply to this area is unique because capillaries branching directly from the central arteriole open into it. Blood is poured directly into this area of splenic tissue. The main function of the marginal zone is to ensure efficient cell exchange between the territories of the white and red pulp.
Characteristics of the red pulp
The red pulp contains inclusions of white pulp surrounded by the marginal sinus. The tissue of the red pulp is structurally represented by:
- Sinusoids, which contain absolutely all formed elements of the blood.
- Spongy formations that are extremely rich in macrophages, lymphocytes, and plasma cells.
The cellular composition of the red pulp has pronounced specific features. NK cells actively migrate here, yet they are practically never found in either the white pulp of the spleen or in lymph nodes. Effector and memory cells predominate among the T and B lymphocyte populations here. Plasma cells migrate into the red pulp from other zones. Due to the accumulation of plasma cells, the red pulp takes a direct part in the effector phase of the humoral immune response.
Anatomical and histological elements of the section
When examining a spleen section in cross and longitudinal directions, general anatomical elements can be visualized: the connective tissue capsule, trabecular artery, trabecular vein, and venous sinuses.
In the microscopic picture of the white pulp, a clear topographical sequence is visible: the central artery runs in the very center, surrounded by the T cell zone — PALS(T). This is followed by the B cell zone — the follicle, inside which a germinal center may be located. This entire structure is bordered by the marginal zone and marginal sinus, beyond which lies the red pulp tissue.
Related topic: Mucosa-associated lymphoid tissue
In addition to the spleen, an important component of peripheral immunity is mucosa-associated lymphoid tissue. Mucous membranes are the most likely route for exogenous aggressive substances to enter the body, which is why they possess well-developed "immunological equipment." The digestive tract bears the heaviest load during biological aggression. The respiratory and urogenital tracts and the small ducts of exocrine glands also contact the external environment.
The immune apparatus of the mucosa is organized as diffuse tissue and organized structures (e.g., Peyer's patches). In the intestinal wall, a Peyer's patch is located in the lamina propria (Lamina propria) and submucosa. It includes primary follicles and follicles with a germinal center, forming a robust inductive site facing the intestinal lumen.
Briefly: answers to topical questions
How is the microvasculature of the spleen structured from the trabecular artery to the venous sinuses?
The microvasculature of the spleen is a strictly ordered vascular system. Blood flow from the trabecular artery to the venous sinuses occurs sequentially:
- Trabecular arteries — run inside connective tissue trabeculae.
- Pulpar arteries — vessels of the white pulp surrounded by a periarterial sheath.
- Central arteries — pass directly through the lymphoid follicles.
- Penicillar arterioles — formed as the vessel enters the red pulp.
- Ellipsoid arterioles — feature a cuff-like thickening made of macrophages and lymphocytes, as well as contractile filaments in endothelial cells.
- Short capillaries — terminal vessels.
In closed circulation, capillaries continue directly into venous sinuses, whereas in open circulation, they empty into the stroma of the splenic cords, from which blood is subsequently collected into the sinuses.
What non-immune functions does the red pulp of the spleen perform?
The red pulp of the spleen performs the functions of blood storage, destruction of aged formed elements, and iron reutilization.
- Storage function — accumulation of a certain volume of blood, erythrocytes, and platelets in the venous sinuses and splenic cords with the ability to release them into the general circulation.
- Destruction of aged cells — macrophages of the splenic cords recognize, capture, and phagocytose old or damaged erythrocytes and platelets.
- Iron reutilization — following the destruction of erythrocytes, macrophages capture iron ions and transfer them to erythroblasts for re-incorporation into hemoglobin synthesis.
What tissue forms the stroma of the white and red pulp of the spleen?
The stroma of the white and red pulp of the spleen is formed by reticular tissue, which belongs to connective tissues with special properties. It forms the supporting framework of the organ and consists of two inextricably linked components:
- Reticular cells — stellate cells that form a reticular structure.
- Reticular fibers — fibrous elements that complement the cellular network.
Developing blood cells, lymphocytes, macrophages, and numerous blood vessels are located within the meshes of this network. The outer border of the lymphoid nodules of the white pulp is also represented by a layer of reticular cells.
What structural features of the endothelium and basement membrane are characteristic of the venous sinuses of the spleen?
The venous sinuses of the spleen are capillaries of the perforated (sinusoidal) type, the structure of which is adapted for blood storage and cell migration. They are characterized by the following features:
- Endothelium — represented by spindle-shaped cells. When the sinus is stretched, large through-holes (slits or fenestrae) form between endothelial cells.
- Basement membrane — discontinuous, containing through slit-like pores.
- Outer layer — the sinus wall is thin, completely lacking pericytes.
This structure allows for bidirectional migration of blood cells between the lumen of the sinus and the reticular tissue of the splenic cords.
To memorize the layers of the white pulp from the vessel to the edge, use the rule "C-P-P-F-M": Central artery → PALS (T cells) → Intermediate space → Follicle (B cells) → Marginal zone.
Class and exam questions
What is the difference between splenic follicles and lymph node follicles?
The differences lie only in minor details. Overall, the patterns of formation, presence of a germinal center, and function (as niches for lymphocytes) are completely identical.
Where are T cells localized in the spleen?
The main thymus-dependent zone is the periarterial lymphatic sheath (PALS), which is directly adjacent to the central artery in the white pulp.
Which cells predominate in the spleen: T or B lymphocytes?
Quantitatively, B lymphocytes predominate over T cells in the spleen.
What is the main immune function of the red pulp?
Due to the presence and accumulation of plasma cells, the red pulp takes an active part in the effector phase of the humoral immune response.
What else to study on this topic
- Organized tissue structures and diffuse lymphoid tissue of mucous membranes
- Mechanisms of formation of the Peyer's patch inductive site
- Cellular composition of the intestinal epithelial layer and lamina propria
- Migration of plasma cells into the red pulp
- Specific functions of marginal zone MZB cells
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